Gravity installed anchor

ABSTRACT

A gravity installed anchor including a metal plate provided with one or more connection points for extending an anchor line from a front face of the plate. The gravity installed anchor preferably includes an elongated shank connected to the plate and arranged for being directed along the plate during the gravity installation process, the elongated shank provided with a nose portion with a nose. The plate may be rotated to an angle relative to the shank after being gravity installed in the seafloor.

The present invention is a gravity installed anchor and a method forinstalling such an anchor. The field of use is for mooring heavy marineconstructions such as semisubmersible drilling platforms, petroleumproduction vessels or similar vessels. A gravity installed anchor isinstalled by connecting it to a mooring line, using ainstallation/retrieval line to lower it to a desired height above theseafloor, and releasing the anchor to fall and penetrate to a desireddepth below the seafloor into the sediments. Then the anchor line may beput in tension.

BACKGROUND ART

Gravity installed anchors usually comprise a long, vertical centralshank, i. e. a weight stem; a massive shank comprising a solid steelcylinder or steel pipe filled with a heavy material such as steel grit,or a shank built of a massive steel plate structure, and provided withsymmetrically arranged so-called “flukes”, i.e. short laterallyextending steel fins which extend along either the upper portion of thestem, or both the upper and lower portion of the stem. The fins arearranged for guiding the stem during the fall through the sea and intothe relatively soft seabed. An attachment point for an anchor line isarranged either in the top stern end of the anchor or in a rotating ringabout the stem. Typical weight ranges for such anchors are from 15 to 45tons.

US patent application US20060107886 describes an anchor for mooringlarge structures to a sea bottom, with an elongated central body withseveral channel members radiating outward from the central stem, eachwith a channel. A plurality of nose and tail plates are disposed withinthe channels and hingedly connected to the central body, so that theplates may be swung in or out from the central body, and fixed by pinsinserted in mating holes. A load ring is arranged encircling the centralbody and the load ring is provided with an arm for linking to an anchorline.

WO2009/105630 describes a method and an apparatus for placing gravityinstalled anchors under offshore conditions for mooring structures tosuch anchors. In an embodiment a gravity install anchor has an outwardextending load arm which is rotatable with respect to the anchor'svertical axis. A load line is attached to the load arm, and a subseaconnector is attached to the remote end of the load line. The subseaconnector is held in a frame at the seafloor and arranged forsimplifying the connection. the gravity install anchor and the mooringassembly is lowered from the anchor handling vessel on a lowering lineand released from a release hook at the end of the lowering line.

International patent application PCT/BR97/000044 Petrobras, published asWO98/08734 describes a plate-type anchor with a flat rectangular platepointed in a front end and with laterally arranged flanges with aninclination to the principal plate. At the rear end of the rectangularplate is a cut-out to allow fitting of a vertical injector pole for useduring the launching of the anchor plate, said injector pole retractableafter installation. Having the large area of the anchor plate at thelower end and the trailing injector pole above, that arrangement mayprove unstable during the drop from the required elevation above the seabed. Further, the launching of the device according to WO98/08734requires two vessels which is a disadvantageous use of resources.

International patent application PCT/NL2002/000509 “Sheet anchor”published as WO2004/011327 describes an anchor plate to be hingedlymounted at its central portion to an upwards extending vertical splitpipe with a threaded-in anchor chain before being stabbed into theseabed. The sheet anchor of WO2004/011327 requires a vibratory hammerfor liquidifying the soil in order to penetrate into the seabed, and isas thus not a gravity install anchor. After launching the split pipe isremoved and the anchor chain is tensioned.

U.S. Pat. No. 6,598,555 “Marine anchors” describes a plate-shaped anchorprovided with a vertically arranged rigid stack of weights forming aweight pile. The weight pile is driven down into the seabed by thegravity force of the weight pile, whereby the weight pile is removed andthe anchor loaded by an attached anchor line. However, the weight of theweight pile is not sufficient to drive the anchor of U.S. Pat. No.6,598,555 to the required depth, and in FIGS. 22-24 it is shown that theweight pile anchor has to be pulled in a complex sequence in order topull out the pile vertically and then pull the anchor plate laterallyand downwards to the required depth. The device of U.S. Pat. No.6,598,555 is thus not purely a gravity install anchor. The outcome ofoperating such a complexly arranged anchor pile and plate in a marineoperation may be unpredictable.

PROBLEMS RELATED TO BACKGROUND ART

The symmetrical gravity installed anchors of the background art havegenerally symmetrical properties about their vertical axis. They may beloaded generally to the same amount in all azimuthal directions. For afour-finned anchor two of the fins may extend in a direction generallynear a transverse plane relative to the anchor line direction, pleasesee FIG. 5, and provide a horizontal retention capacity in thesediments, while the area of the two other orthogonally arranged finscontribute less or not at all to the horizontal retention capacitybecause they are more or less in-line with the anchor line direction.The area of half of the fins is thus not fully exploited in thebackground art. Further, much of the mass is in the generallycylindrical shank and the steel mass contributes comparatively less tothe retention capacity in the transverse direction than it would havedone if formed as fins.

For gravity installed anchors having the anchor line attached in theupper, stern portion there is a risk of inadvertently pulling the anchorback out of the sediments during heavy loading because a large componentof the anchor line force may be in the direction of the stem, andanother, orthogonal component causing an overturning moment i.e.rotating the anchor so that a greater portion of the load is transferredin the direction of the stem.

BRIEF SUMMARY OF THE INVENTION

Some of the disadvantages of the background art i.e. three- orfour-finned gravity installed anchor, may be significantly reduced ifusing an anchor according to the present invention. The invention is, ina material aspect, a gravity installed anchor (1) comprising a metalplate (22) with one or more connection points (8) on a front face (221)of said plate (22) for an anchor line (17). The massive metal plate (22)is arranged for being gravity installed in sedimentary layers under theseafloor. The plate (22) is arranged for being installed transverse tothe extension of the anchor line (17) direction when in its operativestate.

In an advantageous embodiment of the invention the gravity installedanchor according to the invention comprises an elongated shank (0)connected to the plate (22) and arranged for being directed along theplate (22) during the gravity installation process, the elongated weightstem (0) provided with a nose portion (5) with a nose (51). In a furtheradvantageous embodiment of the invention a hinge, preferably centrallyarranged on the plate, may form the link between the plate and the stem,so as for the plate to be able to rotate about the hinge when the anchorline is pulled taut.

Thus, the anchor according to the invention may be defined as a gravityinstalled anchor (1) comprising a main anchor plate (22) with one ormore connection points (8) for extending an anchor line (17) from afront face (221) of said anchor plate (22), and with an elongated shank(0) provided with a nose portion (5), said elongated shank (0) connectednear its top stem portion (6) to said anchor plate (22) and extendingnear vertically downward in its installation state and arranged forbeing directed along said anchor plate during a gravity installationprocess,

characterised by

-   -   said anchor plate (22) pivotally attached to a hinge (61) on        said shank (0),    -   said hinge (61) having a generally horizontal axle (62) for        allowing rotation of said anchor plate (22) relative to said        shank (0) in its installed state, with said shank extending        downwards out from an opposite rear face (222) relative to said        front face (221) of said anchor plate (22).

In an embodiment of the invention the gravity installed anchor (1)further comprises that said hinge (61) is arranged near a centre of saidanchor plate (22).

In an embodiment of the invention the gravity installed anchor (1),further comprises that said hinge (61) is arranged near a central partof an upper portion of said anchor plate (22). An embodiment of theinvention may constitute an intermediate arrangement with the hinge (61)arranged somewhere between the centre of the plate (22) and the centralpart of the upper portion of the plate (22).

According to a method aspect of the invention, it is a method foranchoring using a gravity installed anchor (1), comprising:

-   -   an anchor with a massive metal plate (22) with a hinged shank        (0) extending downwards and an installation connection point (9)        at a top end;    -   connecting an anchor line (17) to one or more connection points        (8) at a front face (221) of said plate (22);    -   attaching an installation line (172) to said one or more        installation connection points (9);    -   using said installation line (172), lowering said anchor (22) to        a desired elevation (h) above a predetermined position (x, y) on        the seafloor;    -   releasing said anchor from said elevation (h);    -   letting said anchor drop and penetrate said seafloor due to        gravity;    -   tensioning said anchor line (17) along said predetermined line        of tension;    -   connecting an opposite end of said anchor line (17) to a vessel        to be anchored.

ADVANTAGES OF THE INVENTION

The anchor according to the invention may be oriented with the plate(22) generally transverse to the desired direction of the anchor line'soperative direction, which is usually determined during the planning ofthe mooring operation. This helps achieving a high retention capacity inthe desired direction. An anchor for this use wherein a major proportionof the wings are directed normal to the load direction will providebetter horizontal retention capacity.

The distribution of the anchor's mass on one single large plate with astem instead of distributing the same mass on a single stem or a stemwith three or more smaller flukes will increase the available anchorplate area available for holding the tension in the anchor line ifaligned normal to the loading direction.

The anchor of the invention, having a large two-winged heavy anchorplate and a weight stem extending below and parallel with the anchorplate, is completely stable during the gravity installation due to themass distribution relative to the wing area distribution. The said shank(0) has weight for leading, and stabilising during the drop. The shankforms a spearhead during gravity installation. The invention is distinctfrom those of the prior art in which a large area anchor plate isarranged at the lower part of a trailing weight pile, which is lessstable.

The anchor according to the invention may be rotated about a horizontalaxis after gravity installation, during the tightening of the anchorline. Thus the retention capacity may be further adjusted to have thehighest retention capacity in the direction of the taut anchor line. Thehinged shank (22) is not preventing said plate (22) from rotating aftergravity installation, thus the hinge link between the shank and theplate advantageously reduces the torsion moment required for thispivoting operation for the plate. Further, the anchor according to theinvention has a reduced risk of being pulled out of the sedimentsbecause the anchor's plate will seek to arrange itself orthogonal to theanchor line when taut, and there will be no or little force component inthe “easy” pull-out direction i.e. parallel to the anchor plate (22).

In an advantageous embodiment of the invention with the connectionpoints (8) slightly offset downwards in their elevation on the plate(22). The connection points may be arranged slightly below thegeometrical centre of the plate (22), so as if the anchor is pulled witha force past its holding capacity, to enable the anchor to be tilted toan angle which will make it penetrate deeper into the sediments, therebyattaining a greater holding capacity.

FIGURE CAPTIONS

The anchor according to the invention is illustrated in the attachedfigures, in which:

FIG. 1 illustrates three different perspectives of an anchor accordingto the invention:

FIG. 1 a is a side elevation view in the plane of the main plate of anembodiment vertically oriented anchor according to the invention, with afront face for being oriented towards an anchor line directed to theright, and with a shank, a weight stem directed downwards.

FIG. 1 b is an isometric view as seen about 45 degrees out of the planeof the same, and showing an opposite rear face relative to the frontface. A hinge is illustrated in the junction between the shank and therecess in the rear face of the main plate and the weight shank. Theshank is illustrated in the parallel position with the main plate.

FIG. 1 c shows in general a front elevation plane view of the front faceof the main plate of the above-mentioned embodiment of the invention.

FIG. 2 illustrates another preferred embodiment of the invention with ahinged connection between the main plate and the shank.

FIG. 2 a is an isometric view as seen in an elevation and about 45degrees out of the rear face of the main plane of a vertically orientedanchor according to the invention, with the main plate and the weightshank arranged in-line.

FIG. 2 b is an elevation view into the plane of the main plate of thesame.

FIG. 2 c is a top view of the same.

FIG. 2 d is a side elevation view of the same, but with the main plateturned about the hinge point into an operative position with an angle ofabout 35-40 degrees inclination relative to the vertically orientedshank. This corresponds to the operative position shown in FIG. 4 c withthe anchor penetrated to a desired depth and the anchor line with itsbridle lines put under tension.

FIG. 2 e is a top view of the anchor according to the invention with theposition of the top plate turned in the same way as for the view of FIG.2 d.

FIG. 3 illustrates an embodiment of the invention rather similar to theembodiment shown in FIG. 1.

FIG. 3 a is an isometric view of the embodiment with the shank in avertical position and the main plate tilted at an angle with the shankand the vertical line. The second, rear face of the main plate relativeto the anchor line is shown. The tilted plate is stabilised in therotary movement about the hinge by the guide fin through the slot nearthe upper part of the shank or weight stem.

FIG. 3 b is an isometric view similar to the view of the embodiment in atilted position as in FIG. 3 a but from an azimuthal angle about 90degrees to the previous illustration, and showing the front face of themain plate from which the anchor line shall extend.

FIG. 4 illustrates the method of installing the anchor according to theinvention.

FIG. 4 a shows a vertical section of the seafloor and an elevation viewof the anchor lowered in an installation line to a desired elevation (h)above a predetermined geographical position (x, y) on the seafloor. Theplate is directed in an azimuthal direction transverse to apredetermined line of tension of the anchor line (17). The installationline may also be used for pulling the anchor out of the seabed duringretrieval or removal of the anchor.

FIG. 4 b shows the anchor gravity-installed in sediments below the sameseafloor with the installation line and the anchor line extendinggenerally up along a vertical penetration channel formed by the fallinganchor.

FIG. 4 c illustrates a further development from the situation in FIG. 4b. The installation line is now connected to a marker buoy in the sea.The anchor line, here connected via three bridle lines to the connectingpoints on the front face of the anchor's main plate has been tightenedand has turned the main plate into a position nearer to a plane normalto the direction of the anchor line's direction in space.

FIG. 5 shows an elevation and plan view of a typical gravity installedanchor of the background art. The plan view shows a typical load in arandom azimuthal orientation relative to the fins.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The anchor according to the invention is illustrated in FIGS. 1, 2, 3,and 4. The invention is a gravity installed anchor (1) comprising amassive metal plate (22), preferably of steel, with one or moreconnection points (8) on a front face (221) of said plate (22) for ananchor line (17). The plate may be slightly kinked along a central linewhich will be generally vertical during the gravity installation, pleasesee FIGS. 1 and 4. Preferably the convex face of the kinked plate is thefront face, i.e. the face from which the anchor line extends. The anchorcomprising at least the massive metal plate (22) is for being arrangedin a vertical position in the sea before being dropped from a desiredelevation above the seafloor. The anchor with the massive metal plate(22) is arranged for being gravity installed in this way in thesedimentary layers under the seafloor. The plate (22) arranged for beinginstalled transverse to the anchor line's (17) extension when in anoperative state. Thus the orientation of the plate is not random: theanchor according to the invention has a significantly higher retentionforce when loaded by a tension line normal to the plane of the plate(22) than if loaded by a tension line directed in a plane parallel withthe plate. In an embodiment of the invention the plate (22) is taperedin its lower portion to a general V-shape pointing along with the shank.

The connection points (8) may in an embodiment be slightlyasymmetrically located on plate (22) with the one or more lowerconnected bridle lines shorter than higher connected bridle lines, suchthat if the anchor is pulled with a force past its holding capacity, theanchor plate (22) and shank (0) will be rotated to an angle which willmake it penetrate deeper into the sediments thereby attaining a greaterholding capacity.

In an embodiment the plate (22) has a general rectangular shape formingtwo fin portions symmetrically extending about a vertical centralsymmetry line as counted in the vertical operational position of theanchor. The two fin portions may be slightly kinked about the verticalcentral line so that the centre of gravity is at or near the verticalaxis of the anchor. The upper, rear part of the fins may form ahorizontal edge, and the lower, front edge part of the fins may betapered off as illustrated in FIG. 1 c and FIG. 2 b. Preferably a mainpart of the outer edge of each fin is straight and parallel to thevertical central line. The tapering may be extensive so that the shapeof the plate (22) may have a general triangular, downward-pointed shape.

It is envisaged that the anchor may generally be constituted by the mainplate (22) with necessary connection points (8) to the anchor line'sbridle lines (17 a, 17 b, 17 c). Such an embodiment would simply be amain plate (22) similar to what is illustrated in FIG. 1, 2, 3 or 4without a weight stem. In an embodiment of the invention the anchor (1)comprises an elongated weight stem or so-called shank (0) connected tothe plate (22) and arranged for being directed along the plane beforeand during the gravity installation process, such as illustrated in FIG.4 a and FIG. 4 b. The weight shank may be massive or comprise a hollowsection ballasted for example with heavy grit. In a preferred embodimentof the invention the elongated shank (0) is provided with a nose portion(5) with a nose (51). The elongated shank (0) is advantageouslygenerally of cylindrical shape, i.e. rotationally symmetric about alongitudinal axis along a significant portion of the shank. The shank(0) may also be made of vertical plates with a cross-section in ageneral X-shape.

The anchor line's bridle lines (17 a, 17 b, 17 c) should be generally ofequal lengths in order for the taut anchor line arrangement to have theplate oriented orthogonal to the anchor line's direction. In order forthe plate to penetrate deeper in case of being loaded beyond its holdingcapacity in the sediment, the upper bridle line or lines should beslightly longer than the lower bridle line or lines, so as for the plateto be slightly tilted and forced to penetrate deeper to regain hold.

EXAMPLES OF DIMENSIONS

In the embodiments shown in FIGS. 1 to 4 the anchor may have a submergedweight of 15 to 45 tons (15000-45000 kg) or even more. The main plate(22) is a thick steel plate of thickness 50 to 100 mm or more. The steelplate may be massive or laminated from a stack of thinner steel plates.A main purpose of the thick steel plate is to provide a large surfacearea transverse to the direction of the anchor line's direction, and toprovide sufficient mechanical stiffness to the anchor, and to providesufficient weight for the anchor to penetrate to a desired depth belowthe seafloor. The overall height of the anchor may be in the range of 6to 12 metres, and the height of the main plate about 4 to 6 m, with thewidth across the main plate being about 3 to 5 m. The area of the mainplate may be in the range between 8 and 20 m². Such an anchor may havean operational tension capacity of about 300 to 750 tons at 15-20 mpenetrated depth depending on the mechanical properties of thesedimentary layers below the sea floor.

In an advantageous embodiment of the invention the plate (22) ispivotally attached to a hinge (61) near an upper part of the stem (0),preferably at a top stem portion (6) of the shank (0) such asillustrated in the operative position in FIG. 4 c, and in furthermechanical detail in FIG. 1 b, FIGS. 2 b and 2 c, and FIG. 3 a. Thehinge (61) may have a generally horizontal axle (62) for tilting theplate (22) relative to the shank (0). In this manner the plate may betilted away from the vertical position after having been dropped intothe sediments wherein the shank initially is in the gravity installedvertical position. After rotating the plate (22), the shank extends withsome angle out from an opposite, rear face (222) of the main plate (22),as illustrated in FIG. 4 c.

In an embodiment of the invention the hinge (61) is arranged near acentre of said plate (22). The hinge may comprise a short robust axlethrough massive hinge eyelets arranged on top of the stem near thecentre of the plate (22) as shown in FIG. 1 b and FIG. 3 a.

In an embodiment of the invention the gravity installed anchor (1),further comprises that said hinge (61) is arranged near a central partof an upper portion of said anchor plate (22). An embodiment of theinvention may constitute an intermediate arrangement with the hinge (61)arranged somewhere between the centre of the plate (22) and the centralpart of the upper portion of the plate (22).

As illustrated in FIGS. 1 b, 3 a, and also in FIG. 2 b, the gravityinstalled anchor (1) according to the invention may have a recess ofgenerally concave or V-shape (63) for receiving at least an upper part(6) of said shank (0) in said rear face (222) of said plate (22). Therecess (63) illustrated in FIGS. 1 b and 3 a may correspond to a ridge(64) on the opposite front face (221) as shown in FIG. 1 c.

The ridge and recess (63, 64) along the main plate (22) increases themain plate's bending stiffness about a horizontal axis through the mainplate. This allows increasing the area of the main plate (22) ascompared to an allowable area of a plain plate without any such ridge orrecess, thus increasing the bearing capacity of the installed anchor. Aridge may also be arranged on the rear face. Further, the recess allowsintegration of the weight shank during installation in order to reducethe cross-section area of the descending anchor so as for reducing theresistance of the anchor in the drop phase in the water and down throughthe sediments.

In an embodiment of the invention illustrated in FIGS. 1 a, 1 b, and 3 athe recess (63) or at least the lower portion of the main plate (22) isprovided with one or more guide fin or plate (224) extending at rightangles to the hinge's (61) axis and parallel to a slot (225) in or alongthe upper stem portion (6). This slot is for receiving said guide plate(224) so as for stabilising the weight stem and the main plate duringthe gravity installation process with the fall through the water and thepenetration through the sediments, and also for stabilizing the mainplate relative to the stem during the subsequent rotation whentightening the bridle lines, please see below. The guide plate (224) maygo through the shank as shown or may comprise two guide plates arrangedin parallel with a separation equal to the local diameter of the shanksuch that the shank is not split or weakened in any way.

In an embodiment of the invention the number of said connection points(8) is at least two. The connection points (8) are preferably arrangedon either sides of the vertical central line of the plate (22), which issaid ridge (64) if present, the connection points (8) for beingconnected to two bridle lines (17 a, 17 b) of said mooring line (17).This arrangement of at least two bridle lines is for enabling to adjustand stabilize the azimuth of the anchor using the mooring line (17)].

If the number of said connection points (8) is three, and at least oneof the bridle lines is arranged below the tilting point of the plate,the plate may be forced to rotate by tensioning the anchor line such asshown in FIG. 4 c. The connection points (8) may in such an embodimentbe arranged generally in a triangular pattern about the centre of saidplate (22), two of said connection points (8) arranged on either sidesof said ridge (64) for being connected to first and second bridle lines(17 a, 17 b) of said mooring line (17) and a third connection pointarranged on said ridge (64) for being connected to a third bridle line(17 c) of said mooring line (17).

The connection points (8) may be formed as one or more of the following:

-   -   eyelets welded to the first, front face;    -   holes through which the mooring line passes and is fixed at the        rear face;    -   slits with through eyelets to the front face, the eyelets        attached to the second, rear face. Ways of attaching the mooring        line to the plate may be envisaged by the person skilled in the        art.

Method According to the Invention

The anchor may be installed as follows:

-   -   An anchor at least comprising a massive, rigid metal plate (22)        with an installation connection point (9) for an installation        line, preferably near a top edge end, is provided from a surface        vessel, please see FIG. 4 a.    -   An anchor line (17), usually provided with two or more bridle        lines (17 a, 17 b, 17 c) is connected in a suitable way to one        or more corresponding connection points (8) at a front face        (221) of the plate (22).    -   An installation line (172) is attached to the installation        connection points (9);    -   Using said installation line (172), the plate (22) is lowered to        a desired elevation (h) above a usually predetermined position        (x, y) on the seafloor, such as is the situation in FIG. 4 a.    -   The plate (22) is then preferably directed in an azimuthal        direction transverse to a predetermined line of tension of said        anchor line (17), as this operation is necessary if the gravity        installed anchor shall be used in a predetermined pattern about        a desired mooring position. One may possibly correct the        azimuthal direction of the plate (22) to a transverse direction        of the anchor line after gravity installation if the sediments        allow so.    -   The plate (22) is then released from said elevation (h);    -   The plate (22) will then drop and penetrate said seafloor due to        gravity, and will stop in the situation illustrated in FIG. 4 b.        The plate will generally now be nearly vertical, aligned with        the shank. The installation line may still be connected to the        tail of the anchor.    -   The anchor line (17) is then put under tension along the above        mentioned predetermined line of tension, please see FIG. 4 c.        The tensioning may be done immediately or completed with the        subsequent final step below.    -   Finally, an opposite end of said anchor line (17) is connected        to a vessel to be moored, or connected to a buoy for being        prepared to connect to the vessel, such as a petroleum drilling        or production platform, if the vessel to be moored has not yet        arrived.

A floating or subsea marker buoy (173) may be attached to theinstallation line (172) for being used for retrieving the anchor ifdesired.

According to the method of the invention the anchor line (17) is putunder tension so as for tilting the plate (22) towards an orthogonalposition relative to said anchor line (17), please see FIG. 4 c. Asmentioned above the anchor line's connection may be arranged with thelower bridle line slightly shorter so as for forcing the anchor toplough deeper into the seabed if its present holding capacity isovercome.

A fixed shank will, due to a longer vertical extension of the combinedstem and plate, require a significantly higher overturning moment thanwhat may be available set up by the anchor line's (17) bridle lines (17a, 17 b, 17 c). An anchor with no shank (0) or a hinged shank (0) mayrequire a higher moment than what may be available set up by the anchorline's (17) bridle lines (17 a, 17 b, 17 c).

The above discussed anchor of the invention may be used to make amooring array of 8 to 12 or more anchors arranged in a pattern about acentral predefined mooring location for a marine floating structure suchas a drilling platform or petroleum production platform.

Penetrometer

Having predetermined weights of gravity installed anchors may incur thatall anchors are designed according to the heaviest anchor required foran operation. It may be advantageous to vary the mass or the penetrationdepth or fluke area of an anchor by testing one or more drop locationsusing a penetrometer model anchor of significantly reduced size. Thismay advantageously be conducted before determining the type of anchorand its size. A method for installing a gravity installed anchor (1) maythus comprise the following steps:

-   -   providing a penetrometer (100) of reduced dimensions and with        similar design and penetration properties as compared to a real        size gravity installed anchor;    -   lowering said penetrometer to a controlled elevation above the        seafloor near a selected anchor installation location;    -   releasing said penetrometer to penetrate said seafloor;    -   measuring at least a penetration depth of said penetrometer, and        preferably providing sediment samples and/or consolidation data;    -   if possible, measure retention capacity after having tensioned        the anchor line    -   configuring a gravity installed anchor according to the        measurement above by conducting one or more of the following        actions:        -   determining the release drop elevation for an available            gravity installed anchor;        -   selecting an anchor between available different gravity            installed anchors;        -   designing a gravity installed anchor based on said            measurement;        -   ballasting an existing gravity install anchor to higher or            lower weight based on said measurement;        -   providing a gravity installed anchor of the so determined            design;

The penetrometer (100) may be measured for its penetration depth and itmay also carry instruments for measuring sediment properties such asconsolidation and shear resistance, and it may comprise a sediment coresampler. The penetrometer could be dropped and retrieved using theinstallation line.

Advantages of Using a Penetrometer:

-   -   One may prepare each anchor for each desired anchor installation        position. Each anchor may thus be prepared or selected to reach        a required minimum penetration depth. As the sediment conditions        may vary from one position to another, this may save the number        of anchors, reduce the required size of each anchor, determine        the type of each anchor, determine the drop elevation of each        anchor, or the ballast volume of each anchor. The method may        also prevent anchors from ending up deeper than required, thus        simplifying the tensioning of the anchor line and easing the        adaptation of the anchor line [anchor chain] length for each        anchor line.

1. A gravity installed anchor including a main anchor plate with one ormore connection points for extending an anchor line from a front face ofsaid anchor plate and with a rear face opposite to said front face, anelongated shank connected at hinge to said anchor plate and arranged forextending near vertically and parallel with said anchor plate duringgravity installation, wherein said hinge arranged for allowing rotatingsaid anchor plate away from a vertical plane and said shank aftergravity installation, said shank extending downwards from said hinge. 2.The gravity installed anchor of claim 1, said hinge arranged at a topstern portion of said shank.
 3. The gravity installed anchor of claim 1,said hinge having a generally horizontal pivot axis.
 4. The gravityinstalled anchor of claim 1, a horizontal rotation axle forming saidhorizontal pivot axis.
 5. The gravity installed anchor of claim 1,further comprising that said hinge is arranged near a centre of saidanchor plate.
 6. The gravity installed anchor of claim 1, furthercomprising that said hinge is arranged near a central part of an upperportion of said anchor plate.
 7. The gravity installed anchor of claim1, said plate being tapered in its lower portion to a general V-shape.8. The gravity installed anchor of claim 1, the number of saidconnection points being at least two, each said connection points forbeing connected to corresponding respective bridle lines of said mooringline.
 9. The gravity installed anchor of claim 1, provided with a recessfor receiving said shank in said rear face of said anchor plate.
 10. Thegravity anchor of claim 9, said recess corresponding to a ridge on saidfront face.
 11. The gravity installed anchor of claim 1, said anchorplate provided with one or more guides such as a guide plate extendingat right angles to said hinge's axis.
 12. The gravity installed anchorof claim 11, said guide plate parallel to a slot along part of saidshank, said slot for receiving said guide plate or plates.
 13. Thegravity installed anchor of claim 1, at least two of said connectionpoints arranged on either sides of a vertical central line on saidanchor plate.
 14. The gravity installed anchor of claim 1, the number ofsaid connection points being three.
 15. The gravity installed anchor ofclaim 14, said connection points arranged in a triangular pattern aboutnear a centre of said anchor plate, one of said connection pointsarranged near said vertical central line on said anchor plate for beingconnected to a bridle line of said mooring line.
 16. The gravityinstalled anchor of claim 1, provided with an installation connectionpoint for an installation line near a top end of said anchor plate. 17.The anchor of claim 1, said plate being massive.
 18. The anchor of claim1, said plate being laminated from a number of thinner steel plates. 19.A method for anchoring using a gravity installed anchor whereinproviding a massive metal plate with a downwards extending stem hingedto said plate and an installation connection point at a top end;connecting an anchor line to one or more connection points at a frontface of said plate; attaching an installation line to said installationconnection point; using said installation line, lowering said plate to adesired elevation (h) above a predetermined position (x, y) on theseafloor; releasing said plate from said elevation (h); letting saidplate drop and penetrate said seafloor due to gravity; tensioning saidanchor line along a predetermined line of tension; connecting anopposite end of said anchor line to a vessel to be anchored.
 20. Themethod of claim 19, tensioning said anchor line so as for tilting saidplate towards an orthogonal position relative to said anchor line. 21.The method of claim 19, wherein after the step of lowering the plate toa desired elevation (h) above the seafloor, directing said plate in anazimuthal direction transverse to said predetermined line of tension ofsaid anchor line.
 22. The method of claim 19, verifying the direction ofsaid plate after letting said plate penetrate said sediments.
 23. Amethod for installing a gravity installed anchor according to claim 19,including providing one or more gravity installed anchors; providing apenetrometer of reduced dimensions and with similar design andpenetration properties as said one or more gravity installed anchor;lowering said penetrometer to a controlled elevation above the seafloornear a selected anchor installation location; releasing saidpenetrometer to penetrate said seafloor; measuring at least apenetration depth of said penetrometer; configuring one of said gravityinstalled anchor according to the measurement above by one or more ofthe below actions: determining the release drop elevation for thegravity installed anchor; selecting an anchor between availabledifferent gravity installed anchors; designing a gravity installedanchor based on said measurement; ballasting a gravity installed anchorto higher or lower weight based on said measurement.
 24. The methodaccording to claim 23, using a sediment sampler in said penetrometer forproviding sediment samples.